All motors, generators and transformers employ a stator core comprising a stack of compressed laminations. Means used to compress the laminations include clamping bolts or keybars as in U.S. Pat. Nos. 3,652,889; 4,564,779, throughbolts as in U.S. Pat. No. 7,223,920; 2010024461, welding as in U.S. Pat. No. 4,485,320, bonding as in U.S. Pat. Nos. 4,085,347, 4,123,678, cleating or interlocks as in U.S. Pat. No. 5,918,359.
Small and medium sized electric machines often employ simple round-sectioned steel bolts to compress the laminations. Large dynamoelectric machines on the other hand employ custom bolts with a dovetail, termed keybars, to compress the laminations and attach them to the frame rings. The term “keybar” used herein refers to both round bolts and keybars; they pass through open slots in the outer periphery of laminations. Throughbolts on the other hand refer to long round bolts inserted via through holes inside laminations, and are used to apply additional compression. Irrespective of the size or shape of clamping bolts, their outer surface (termed “keybar-contact”) makes an electrical contact with laminations and other attached parts such as flanges and frame rings. This keybar-contact acts as a conduit for transmitting hazardous eddy currents produced by the machine. In prior art, keybar contact has practically zero-resistance; no attempt is made to increase its surface resistance. Such prior-art keybar that makes zero resistance contact with other parts is termed “uncoated keybar” herein. Such uncoated keybar does not have surface modifiers or coatings on its surface that can alter its surface resistance.
FIG. 1A shows two laminations 20, 21 joined by coated area 16 suspended by an uncoated keybar 10. The two laminations 20, 21 are shorted at point s in the insulative coating 16. Shorts can occur due to burrs on edges, defects in the insulated surface or interlaminar microwelds at high points of contact. FIG. 1A also depicts an uncoated keybar 10 making electrical contacts at a and e with laminations 20, 21. The uncoated keybar 10 aids circulation of destructive Fault Currents as follows. The alternating main flux φθ induces a small eddy voltage Vθ=dφθ/dt in the laminations. This causes Fault Current If to flow through laminations 20, 21 along the loop absde in the direction indicated by the arrows. In the lamination 20 containing leg ab, it flows radially inward from the keybar-contact a for a distance equivalent to the length of the shorted span. In the shorted segment bsd, it flows axially through the short s. In the lamination 21 containing leg de, it flows radially outward until it reaches the keybar-contact e. In the uncoated keybar 10 containing the leg ea, the fault current flows from key-bar contact e to the keybar-contact a. The uncoated keybar shorts with laminations at a and e enabling closure of the Fault Current loop. The electrical resistance to eddy current flow is low, leading to large fault currents. Large currents through a small area at s cause high heat load resulting in hot spots. If several hot spots are concentrated in a localized area, they can interconnect and grow, causing a core failure. Thus in prior art, excessive Fault Currents, aided by the uncoated keybar, can lead to hot spots and core failure as discussed in U.S. Pat. Nos. 8,289,031, 8,179,028, 4,494,030; 4,573,012; 4,996,486 and 6,791,351, Edmonds 2007.
FIG. 1B describes how prior-art produces destructive Keybar Currents, aided by the uncoated keybars. It depicts a typical “keybar grid” at the core-end, comprising two adjacent keybars 10, 11 plus electrically connected flange or frame ring 27 and an adjacent frame ring 28. At contacts p, t the uncoated keybar is electrically shorted to the flange 27 while at the contacts q, r it is electrically shorted to frame ring 28. When the machine is overfluxed (viz., operated at unloaded open circuit condition, loaded leading power factor condition, short-circuit condition etc.) the core-end laminations are nearly saturated as they are trying to carry too much flux—normal main flux, fringe flux, plus abnormal overflux. As a result, some flux leaks into the air surrounding the outer core periphery. This leakage flux has a rotating radial component φr, which induces current in several electrical paths. FIG. 1B shows a particular path through the keybar grid. The rotating flux φr cuts two successive keybars 10, 11 so induces a small voltage Vk in them. This drives Keybar Current Ik, which circulates along pqrt path in the direction indicated by arrows. In the uncoated keybar 10, it flows axially from keybar-contact p to keybar-contact q. In the frame ring 28, it flows peripherally from keybar-contact q to keybar-contact r. In the uncoated keybar 11, it flows axially from keybar-contact r to keybar-contact t. In the flange (or frame ring) 27, it flows peripherally from keybar-contact t to keybar-contact p closing the loop. If there are axial or peripheral shorts between laminations, some of the Keybar Current can also flow through the laminations and damage them. Prior art suffers from such destructive Keybar Currents aided by uncoated keybars as discussed in several patents, viz. U.S. Pat. Nos. 3,652,889; 6,429,567; 6,713,930 and 6,720,699.
In a similar fashion, hazardous throughbolt currents can circulate in machines employing uncoated throughbolts. A typical “throughbolt grid” comprises two adjacent uncoated throughbolts attached to flanges on both ends. The uncoated throughbolt contacts both flanges as well as laminations. The flux hitting the uncoated throughbolts has a rotating radial component that induces a small voltage in the throughbolts. This results in throughbolt current It that circulates in all the throughbolt grid parts. In the uncoated throughbolts, this current flows axially from a first flange end to the second flange end. In the flanges, it flows peripherally from one throughbolt to adjacent throughbolt via the throughbolt contacts. If there are shorts within the core, some of the throughbolt current can also flow through the laminations damaging them. It is apparent that in prior-art, an uncoated throughbolt detrimentally facilitate flow of destructive throughbolt currents.
Thus, the uncoated keybars or throughbolts used by prior-art has several damaging consequences. First, they support circulation of perilous Fault Currents If, Keybar Currents Ik and throughbolt Currents It. Excessive fault currents can cause hot spots. Excessive hot spots concentrated in a small area can result in core failure. To avoid core failure, one might be forced to derate the machine. Excessive fault currents can also repel the main flux, thereby reducing the power output. They can also increase core losses or reduce efficiency. Excessive keybar currents can cause core-end heating or core decompression and subsequent loose laminations. The keybar current is significantly larger than the fault current. Large keybar currents crowding over the outer surface (up to its skin depth) can overheat the keybars, flanges, frame rings etc. An overheated keybar could expand, causing the core to de-compress, resulting in loose laminations. Expanding keybars could also cause thermal stresses in the frame rings. Thus, large keybar currents can damage a machine that contains fully insulated laminations that have no shorts. They also limit the leading power factor regime or cause derating of the machine. Some keybar current can also leak via laminations and damage them also. To prevent all such severe damages, there is a need to reduce all—fault current, keybar current and throughbolt current simultaneously. Prior art employed several methods to limit damages: these include recoating, fault detection, flux shielding, insulating the keybar etc.
First, the recoat method involves burr-grinding laminations followed by a relatively a thick recoat per Coombs 2001. A recoat is believed to further reduce the risk of short-circuits arising from burrs. However, recoating requires installation of a recoating plant and maintaining strict control for optimal hardness of the recoat. Since a recoating plant requires large capital investment and its operation demands expensive quality control, the recoat method increases the overall cost of the machine.
Second, the core fault detection methods have been ubiquitously employed to detect core faults. These include a high-powered Ring Flux test and low powered EL-CID test EL-CID that stands for “Electromagnetic Core Imperfection Detector” (EL-CID) as disclosed in UK Patent 2,044,936. Both try to locate the core sections containing the shorts or faults. Once a fault area is identified, the technicians attempt to replace or repair the shorted laminations. However, these devices are expensive and require skilled and experienced operators. Even then, the results can sometimes be misleading. In fact, a 2004 EPRI publication 1009855 entitled “Generator core overheating risk assessment” reports on p. 5-2 that ELCID could lead to attempting to repair defects that are not there; they further state that there did not appear to be a correlation between the intended severity of the defects and the current flow measured by EL-CID. Such documented remarks cast doubts on the effectiveness of some core fault-detection devices.
Third, the insulated keybar method uses an insulated keybar to eliminate Fault Current. This insulation is provided either by a loose insulative sleeve over an uncoated keybar as in U.S. Pat. No. 4,494,030, or an insulative strip wedged into dovetail slot as in U.S. Pat. No. 7,202,587 or a heat shrinkable tubing wrap over an uncoated throughbolt as in U.S. Pat. No. 6,949,858. Such insulation electrically cuts off the keybar contact in the Fault Current path so zeroes the Fault Current. To avoid ground fault risk, the keybar-contact should be non-insulative. The keybar-contact is required to electrically connect the stator core to the frame in order to close the ground current loop. An insulated keybar breaks this grounding loop and subjects the machine to ground fault risks.
Fourth, the flux shield method uses a conductive shield, shunt etc to reduce the Keybar Current. Conductive straps attached to the dovetail slot that short with keybar were proposed in U.S. Pat. Nos. 6,462,457 and 6,720,699. Conductive wires connecting all these straps were also proposed in U.S. Pat. No. 6,429,567 to shunt the Keybar Current. A leaf spring to short keybar and laminations was proposed in U.S. Pat. No. 6,548,928. Adjusting the number of keybars, position of keybar relative to phase belt etc was proposed in U.S. Pat. No. 7,113,899. All these means however short the keybar with laminations. However, a shorted keybar closes the path for the destructive fault current and excessive fault currents unfortunately can destroy the core.
Thus, all these prior-art methods to reduce the Fault Current or Keybar Current suffer from serious drawbacks. The recoat method requires large capital and expensive quality control. The core-fault detection method requires significant expertise, expensive equipment and can be misleading. The insulated keybar method fails to ground the core. The flux shield method increases core failure risk. Thus none of the prior art method can protect all components of the stator or prevent core-end heating. In contrast, this disclosure presents a method that simultaneously reduces the Fault Current, Keybar Current and Throughbolt current, thereby preventing core-faults and core-end heating. It also prevents core failure or derating of the machine. It also eliminates the need for core-fault detection tests or recoating the laminations. It also prevents decompression of the core and consequent loosening of laminations.